Object Mating (Relative positioning)
You can do this with my addon, Mesh Align Plus (mating, relative movement/positioning, and other abstract precision transformations are its specialties). 

To mate an object (or a group of objects) to another, just grab a source and a destination plane, then hit apply (to the selected objects). The GIF below shows relative positioning of 4 separate bracket objects, along with some uniform-spacing after the mate operation:

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You can think of the "Source" and "Destination" planes as 3-dimensional guides for your alignment. You can grab these guides from anywhere: a real face, or 3 disconnected verts (or from separate meshes, or the 3d cursor in the advanced tools).

You can apply operators to object(s), or to mesh parts. And, you can apply an operator to completely separate/unrelated objects also (this is what makes multi-object editing/relative positioning in the GIF work).